How late can I transplant Day Lilies so that they will bloom in spring

They can be replanted now before the ground is frozen.

In our area until first frost, usually Dec.

I would replant them now.

Hello there. Cindy here. You could cut them back now. Clip them where the plants meet the soil. Leave the roots alone. Dispose of all the plants and debris. You need to cover your roots to protect them from winter weather. Cover them with mulch, grass cuttings or even cardboard. If you use cardboard, place big rocks on it so that it doesn't blow away. After first frost, transplanting is done. The plants growth will fail. I hope this info helps you. Good luck from Cindy.

Now will be the time